The Bacillus thuringiensis delta-endotoxin is a commercially important pesticide which has specific insecticidal activity on a number of insects. Because it is a biodegradable protein and is not toxic to vertebrates and most non-target insects, it has a low environmental impact. Numerous derivatives are available due to the existence of several genes which encode the delta-endotoxin. As a protein, it has many interesting properties making it ideal for intensive biochemical studies. It is expressed in vivo as a crystal and is developmentally expressed only during sporulation. As a protein toxin, it shares certain properties with other protein toxins, but also has unique features which need to be further clarified. Funds are requested to use genetic engineering and protein engineering techniques to localize the domains of the protein which affect insect specificity, bind to tissue culture cells, and cause cytolytic damage. Changes in the toxin amino acid sequence, which will be performed in this project, are aimed at improving the specificity and activity of the toxin toward certain insect crop pests.